Magnesium-lithium alloys having improved characteristics

ABSTRACT

Magnesium lithium based alloys prepared by mechanical alloying are disclosed.

BACKGROUND OF THE INVENTION

The present application is related to magnesium-lithium based alloys andmethods of their preparation.

Magnesium-lithium based alloys have been recognized as having potentialin aerospace applications since the early 1960's. These alloys have lowdensity along with mechanical properties, weldability and physicalproperties which make them interesting for use in aircraft and aerospaceapplications.

Traditionally, magnesium-lithium alloys have been melted and cast byconventional methods. A problem with producing magnesium-lithium alloysin this way is that lithium and, to a lesser extent, magnesium reactreadily with oxygen and nitrogen in the temperature range of about 650°C. to 750° C. which is required for melting. In addition, the ductilityof these alloys is quite sensitive to low levels of sodium impuritiesrequiring use of high purity lithium.

One approach to protecting the molten alloys from contact with oxygenand nitrogen and the related danger of burning is to use a flux cover onthe exposed surface of the melt. A second method uses an inert gas coverto protect the molten metal. A third method that has been considered ismelting under vacuum. None of these methods are without problems.

Magnesium-lithium based alloys made by conventional routes have severestrength limitations. Alloys have been made containing such elements assilver, aluminum, cadmium or zinc which have high strength, but theprecipitate that imparts high strength is unstable and ages excessivelyeven at room temperature resulting in significant loss of strength.Additionally, alloys made this way are sensitive to stress corrosioncracking. Alloys which are stable at room temperature have low strength.Furthermore, these alloys all have low creep strength.

Additionally, since magnesium-lithium alloys were first seriouslyconsidered for use in aerospace and other applications, the demands onsuch materials have changed and there exists a need for alloys havingimproved properties of yield strength (under compression or tension),ultimate tensile strength, creep strength and thermal stability.

Thus, there is a need for magnesium-lithium alloys having improvedcharacteristics that are prepared by methods which avoid the dangersassociated with traditional ingot metallurgy.

SUMMARY OF THE INVENTION

The present invention is a magnesium based alloy containing lithium and,optionally, aluminum, zinc, zirconium, titanium, calcium, tin, silver,yttrium, cerium, neodymium or mixtures thereof which is prepared bymechanical alloying and has mechanical properties and thermal stabilitycharacteristics which are improved over characteristics of identicalalloys prepared by other methods such as ingot metallurgy.

Such alloys are useful in, for example, aerospace applications.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

The magnesium-lithium based alloys of the present invention preferablycorrespond to the formula:

    Mg.sub.w Li.sub.x M.sub.y I.sub.z

wherein M is an element selected from the group consisting of aluminum,zinc, zirconium, titanium, calcium, silver, tin or mixtures thereof; Irepresents impurities; w represents the weight percent of Mg in thealloy; x represents the weight percent of Li in the alloy; y representsthe weight percent of M in the alloy; z represents the weight percent ofimpurities in the alloy; w+x+y+z=100; the value of w is at least about50 and no greater than about 88; the value of x is at least about oneand no greater than about 50; the value of y is from zero to about 10;and the value of z is from zero to about 5.

In a preferred embodiment, the value of w is at least about 65 and nogreater than about 88; the value of x is at least about 12 and nogreater than about 20; the value of y is at least about 1 and no greaterthan about 15; and the value of z is no greater than about 2. When M ispresent, it is preferably selected from the group consisting ofaluminum, zinc, calcium and silver. It is preferred that M is present.It is more preferred that x is from 13 to 15, most preferably about 14.

Alloys prepared by the process of the present invention include thosecontaining at least 14 weight percent lithium; up to 10 weight percentaluminum, zinc, calcium, tin, yttrium, silver, titanium, zirconium,cerium and/or neodymium, with the balance being magnesium.

The mechanically alloyed magnesium-lithium based alloys of the presentinvention have improved properties of tensile yield strength, ultimatetensile strength, compressive yield strength, hardness and thermalstability when compared to alloys prepared by conventional ingottechnology.

The alloys of the present invention are prepared by mechanical alloying.Mechanical alloying as a method of preparing alloys is in general wellknown. For example, U.S. Pat. Nos. 4,624,705 and 4,758,273 each discussmethods of preparing aluminum alloys. Generally, mechanical alloyingmeans a process wherein powder ingredients are subjected to impacts byan impacting medium so as to cause a multiplicity of particle weldingsand fracturing until the powder ingredients are converted to anessentially uniform powder product. Attritors and horizontal ball millsare examples of means often used for mechanical alloying.

Processing aids such as lithium stearate, zinc stearate, stearic acid,graphite and decanoic acid are preferably used in the process of thepresent invention in an amount effective to prevent or lessen thewelding of the powders to the grinding media or apparatus.

The mechanical alloying of the present invention is done under aprotective atmosphere such as argon or other inert gas or under vacuum.The purpose of the protective atmosphere is to avoid oxidation of thematerials being alloyed. Safety considerations also make it desirable toavoid oxygen, nitrogen and other gases with which the metal powdersbeing alloyed could react explosively.

Other conditions that are important in the mechanical alloying processof the present invention include operating as close to ambienttemperature as possible.

The following examples are provided to illustrate the invention andshould not be interpreted as limiting it in any way. Unless statedotherwise, all parts and percentages are by weight.

In the following examples, the mechanically alloyed alloys were preparedusing the following steps:

(a) commercially available elemental powders of magnesium, lithium andother metals were used;

(b) a processing additive was added to the starting materials in a givenamount;

(c) the starting materials were added to an appropriate vial under aninert atmosphere;

(d) the vial was placed in a shaker mill and shaken for a specifiedtime.

The alloying was conducted at ambient temperature and pressure.

EXAMPLE 1

Using the above procedure wherein from 1-2 weight percent stearic acidand lithium stearate were used as processing additives, the followingalloys were prepared:

    ______________________________________                                        (a)   Mg.sub.80.5 Li.sub.14 Al.sub.1 Zn.sub.4 Zr.sub.0.5                                            (b)     Mg.sub.80 Li.sub.14 Al.sub.1 Zn.sub.4                                         Zr.sub.1                                        (c)   Mg.sub.79.5 Li.sub.14 Al.sub.2 Zn.sub.4 Zr.sub.0.5                                            (d)     Mg.sub.79 Li.sub.14 Al.sub.2 Zn.sub.4                                         Zr.sub.1                                        (e)   Mg.sub.78.5 Li.sub.14 Al.sub.3 Zn.sub.4 Zr.sub.0.5                                            (f)     Mg.sub.78 Li.sub.14 Al.sub.3 Zn.sub.4                                         Zr.sub.1                                        (g)   Mg.sub.86 Li.sub.14                                                                           (h)     Mg.sub.84 Li.sub.16                             (i)   Mg.sub.82 Li.sub.14 Al.sub.1 Zr.sub.3                                                         (j)     Mg.sub.82 Li.sub.14 Al.sub.1 Ti.sub.3           (k)   Mg.sub.70 Li.sub.30                                                                           (l)     Mg.sub.80 Li.sub.20                             (m)   Mg.sub.78 Li.sub.22                                                                           (n)     Mg.sub.65 Li.sub.20 Al.sub.15                   (o)   Mg.sub.75 Li.sub.15 Al.sub.5 Zn.sub.5                                                         (p)     Mg.sub.65 Li.sub.15 Al.sub.10 Zn.sub.10         (q)   M.sub.76 Li.sub.14 Al.sub.10                                                                  (r)     Mg.sub.79 Li.sub.14 Al.sub.7                    (s)   Mg.sub.81 Li.sub.14 Al.sub.5                                                                  (t)     Mg.sub.77 Li.sub.20 Al.sub.3                    (u)   Mg.sub.93 Li.sub.7                                                                            (v)     Mg.sub.91 Li.sub.9                              (w)   Mg.sub.77 Li.sub.19 Al.sub.4                                                                  (x)     Mg.sub.78 Li.sub.16 Al.sub.6                    (y)   Mg.sub.81 Li.sub.14 Ag.sub.5                                                                  (z)     Mg.sub.76 Li.sub.14 Ag.sub.10                   (aa)  Mg.sub.81 Li.sub.14 Ti.sub.5                                                                  (bb)    Mg.sub.76 Li.sub.14 Ti.sub.10                   (cc)  Mg.sub.81 Li.sub.14 Zr.sub.5                                                                  (dd)    Mg.sub.76 Li.sub.14 Zr.sub.10                   (ee)  Mg.sub.79 Li.sub.14 Al.sub.7                                                                  (ff)    Mg.sub.76 Li.sub.14 Al.sub.10                   ______________________________________                                    

EXAMPLE 2

Using the method set forth above, the alloys listed in Table I wereprepared using 1 weight percent stearic acid as a processing additiveand 16 hours as shaking time. The powder was removed from vials and coldcompacted at 50,000 pounds force. The compact was heated to 300° C. andextruded. The extrusion ratio (ratio of cross sectional area of compactdivided by the cross-sectional area of the extruded rod) was 16:1. Therod diameter was 5/16 inches. The rods were then tested to determinetheir properties.

Tensile tests were done according to the ASTM method B557-84 on standardsamples with a 1/8 inch diameter and a length of 0.5 inch. A dynamicextensometer was used to measure strain to obtain the elastic modulusand 0.2 percent offset yield strength. Compression tests were done onright circular cylinders according to ASTM method E9-89. The samplediameter was 0.225 inch and the length was 0.7875 inch. Again, a dynamicextensometer was used to measure strain to obtain the elastic modulusand 0.2 percent offset yield strength. Hardness numbers were measuredusing a Tukon microhardness tester using a load of 100 g. The density ofthe extruded alloys was measured using a helium pycnometer. The resultsobtained are given in Table I below.

                  TABLE I                                                         ______________________________________                                                    DEN.     UTS     TYS   CYS                                        ALLOY       (g/cm.sup.3)                                                                           (ksi)   (ksi) (ksi) % E                                  ______________________________________                                        Mg.sub.86 Li.sub.14                                                                       1.4446   28.97   28.077                                                                              33.166                                                                              7.3                                  Mg.sub.85 Li.sub.14 Al.sub.1.sup.1                                                        1.4577   32.84   31.144                                                                              34.322                                                                              13.9                                 Mg.sub.85 Li.sub.14 Al.sub.1.sup.2                                                        1.3784   17.7    13.309                                                                              13.517                                                                              54.0                                 Mg.sub.83 Li.sub.14 Al.sub.3                                                              1.4490   34.03   31.161                                                                              37.595                                                                              9.0                                  Mg.sub.81 Li.sub.14 Al.sub.5                                                              1.4844   38.57   36.821                                                                              41.068                                                                              7.0                                  Mg.sub.78 Li.sub.16 Al.sub.6                                                              1.4174   35.96   32.732                                                                              39.061                                                                              7.6                                  Mg.sub.85 Li.sub.14 Zn.sub.1                                                              1.4634   28.81   27.822                                                                              33.788                                                                              12.7                                 Mg.sub.83.5 Li.sub.14 Zn.sub.2.5                                                          1.4670   28.31   27.486                                                                              35.975                                                                              14.6                                 Mg.sub.80 Li.sub.14 Zn.sub.5                                                              1.4957   31.87   29.922                                                                              36.113                                                                              9.0                                  Mg.sub.76 Li.sub.14 Zn.sub.10                                                             1.5371   29.41   27.702                                                                              36.898                                                                              9.0                                  Mg.sub.80.5 Li.sub.14 Al.sub.3 Zn.sub.2.5                                                 1.4787   31.84   30.339                                                                              38.479                                                                              4.4                                  Mg.sub.85 Li.sub.14 Sn.sub.1                                                              1.4637   29.5    27.416                                                                              31.654                                                                              8.4                                  Mg.sub.81 Li.sub.14 Ca.sub.5                                                              1.4652   33.95   31.199                                                                              35.969                                                                              9.1                                  Mg.sub.81 Li.sub.14 Y.sub.5                                                               1.4944   26.5    25.5  31.392                                                                              7.0                                  Mg.sub.76 Li.sub.14 Y.sub.10                                                              1.5100   28.98   27.876                                                                              30    7.0                                  Mg.sub.81 Li.sub.14 Nd.sub.5                                                              1.4870   28.22   27.784                                                                              28.22 3.0                                  Mg.sub.76 Li.sub.14 Nd.sub.10                                                             1.5165   NA      NA    30    NA                                   Mg.sub.76 Li.sub.14 Ce.sub.10                                                             1.5405   26.28   25.182                                                                              28.784                                                                              4.7                                  Mg.sub.79 Li.sub.14 Al.sub.7                                                              1.4776   38.45   36.31 43.52 3.5                                  Mg.sub.76 Li.sub.14 Al.sub.10                                                             1.4991   38.68   37.81 53.1  1                                    Mg.sub.81 Li.sub.14 Ag.sub.5                                                              1.4857   35.13   33.81 37.95 12                                   Mg.sub.76 Li.sub.14 Ag.sub.10                                                             1.5371   36.85   35.42 40.32 3.52                                 Mg.sub.81 Li.sub.14 Ti.sub.5                                                              1.4810   26.22   25.4  28.53 7                                    Mg.sub.76 Li.sub.14 Ti.sub.10                                                             1.5065   26.32   25.5  28.48 4                                    Mg.sub.81 Li.sub.14 Zr.sub.5                                                              1.4796   26.76   25.8  28.62 14                                   Mg.sub.76 Li.sub.14 Zr.sub.10                                                             1.5158   25.97   24.87 28.7  14                                   ______________________________________                                         UTS: Ultimate Tensile Strength                                                TYS: Tensile Yield Strength                                                   CYS: Compressive Yield Strength                                               % E: % Elongation                                                             .sup.1 Value presented is average of 10 alloys.                               .sup.2 Not an embodiment of the invention. Alloy prepared by ignot            metallurgy.                                                              

The data in Table I clearly shows the improved characteristics obtainedwhen a magnesium-lithium based alloy is prepared by mechanical alloyingrather than by conventional techniques such as ingot metallurgy.

What is claimed is:
 1. A magnesium lithium alloy corresponding to theformula

    Mg.sub.w Li.sub.x M.sub.y I.sub.z

wherein M is an element selected from the group consisting of aluminum,zinc, calcium, tin, yttrium, silver, titanium, zirconium, cerium andneodymium and mixtures thereof; I represents impurities; w representsthe weight percent of Mg in the alloy; x represents the weight percentof Li in the alloy; y represents the weight percent of M in the alloy; zrepresents the weight percent of impurities in the alloy; w+x+y+z=100;the value of w is at least about 50 and no greater than about 88; thevalue of x is at least about one and no greater than about 50; the valueof y is from zero to about 10; and the value of z is from zero to about5, prepared by mechanical alloying.
 2. The alloy of claim 1 wherein w isfrom 65 to 88; x is from 12 to 20; y is from 1 to 15 and z is no greaterthan
 2. 3. The alloy of claim 1 wherein x is from 13 to
 15. 4. The alloyof claim 3 wherein M is aluminum, zinc, calcium or silver.
 5. The alloyof claim 4 wherein M is aluminum and y is from 1 to
 10. 6. The alloy ofclaim 4 wherein M is zinc and y is from 1 to
 5. 7. The alloy of claim 4wherein M is calcium and y is from 1 to
 5. 8. The alloy of claim 4wherein M is silver and y is from 1 to 5.